Because of their finite reserves, global petroleum resources are running out. With many developing countries currently industrializing, petroleum demand has sharply increased, causing an imbalance between market demand and supply and leading to an era of high oil prices. Furthermore, the reckless use of petroleum has brought about an explosive increase in greenhouse gases, provoking significant environmental problems such as global warming.
Extensive worldwide efforts have long been made to use biomass, which is regenerable and sustainable, as an alternative to petroleum resources. As a result, biofuels, such as bioethanol, biodiesel, etc. and bioplastic monomers, such as lactic acid, propanediol, etc., are successfully produced on an industrial scale and are used as substitutes for transportation fuels or petrochemical materials.
In recent years, intensive attention has been paid to levulinic acid as it has been discovered to be producible via conversion of biomass-derived carbohydrates.
During the dehydration, levulinic acid can be synthesized from hexoses via 5-hydroxymethyl-2-furfural (HMF) as an intermediate or from pentoses via furfural as an intermediate. Of carbohydrates, fructofuranose, an isomeric form of fructose, is most easy to convert into the intermediate 5-hydroxymethyl-2-furfural.
According to a report released by the U.S. Department of Energy (DOE), levulinic acid is one of the platform compounds chosen as “Top Value-Added Chemicals from Biomass”, and can be converted into a wide spectrum of compounds including a monomer for polymers, a herbicide, a medicine, a flavoring agent, a solvent, a plasticizer, an antifreezing liquid, a fuel additive, etc. [(i) J. J. Bozell, G. R. Petersen, Green Chem. 2010, 12, 539-554; (ii) T. Werpy, G. R. Petersen, Top value-added chemicals from biomass volume I—Results of screening for potential candidates from sugars and synthesis gas: U.S. Department of Energy, NREL/TP-510-35523 (2004)]. However, industrial mass production of levulinic acid has not yet been achieved.
Levulinic acid is synthesized by dehydrating biomass-derived carbohydrates under an acidic condition. Typically, the dehydration is conducted by heating a carbohydrate in a 1-10% aqueous solution of an inorganic acid. The inorganic acid may be hydrogen chloride or sulfuric acid.
However, the use of such a homogenous inorganic acid as a catalyst entails problems in industrial application because the acid is difficult to remove from the solution after the dehydration, thus producing a great deal of highly concentrated waste water. That is, a significant portion of the production cost of levulinic acid is allocated to the treatment of the highly concentrated wastewater produced upon the use of the inorganic acid as a catalyst.